This application claims the priority benefit of Taiwanese application serial no. 91106568, filed on Apr. 2, 2002.
1. Field of the Invention
The invention relates in general to an initialization method of a semiconductor memory, and more particularly, to an initialization method of a P-type silicon nitride read only memory (NROM).
2. Description of the Related Art
The flash memory is a kind of electrically erasable and programmable read only memory (EEPROM) with the capability of saving, reading and erasing data multiple times. Moreover, the data saved in the flash memory does not disappear even when the electricity is interrupted. It thus has become one of the most commonly used memory devices for personal computers and electronic equipment.
The typical flash memory uses a doped polysilicon layer to fabricate the floating gate and the control gate. When a memory is programmed, appropriate programming voltages are applied to the source region, the drain region and the control gate, respectively. The electrons then flow from the source region to the drain region via the channel. In this process, some of the electrons tunnel through the tunneling oxide layer under the floating gate and are evenly distributed in the control gate. The electron tunneling effect through the tunneling oxide to the control gate can be divided into two situations. One is called xe2x80x9cchannel hot-electron injectionxe2x80x9d; the other is called xe2x80x9cFowler-Nordheim tunnelingxe2x80x9d. Normally, channel hot-electron injection is applied to program the flash memory, and Fowler-Nordheim tunneling is applied to the side of the source region and the channel region when erasing. However, if the tunneling oxide layer under the floating gate has weak point, a leakage current of the device is easily caused to affect the reliability of the device.
One conventional method of resolving the problem of device leakageuses a charge trap layer to replace the polysilicon floating gate. The material of the charge trap layer includes silicon nitride. An EEPROM with a stacked gate structure made of a silicon oxide/silicon nitride/silicon oxide (ONO) stacked layer is thus formed. Since the material of the charge trap layer is silicon nitride, this kind of EEPROM is called silicon nitride read only memory (NROM). As the silicon nitride layer has the function of trapping charges, the electrons injected into the silicon nitride floating gate are not uniformly distributed. Instead, the electrons are distributed in a Gaussian distribution in the local area of the silicon nitride floating gate. Since the injected electrons are concentrated in a local area of the silicon nitride floating gate, the sensitivity of the weak points is small, and the device leakage does not easily occur.
Another advantage of using the silicon nitride floating includes that the electrons are locally stored in the channel region close to the upper part of the source or drain region. Therefore, while programming, voltages can be applied to the source/drain region at two sides of the stacked gate and the control gate respectively, so that a Gaussian distribution of electrons is generated in the silicon nitride floating gate near another side of the source/drain region. By changing the voltages applied to the control gate and the source/drain region, two groups of Gaussian distributed of electrons, one group of Gaussian distributed of electrons, or non-existence of electrons can be obtained in the floating gate. Therefore, using silicon nitride as the material for fabricating the floating gate, four statuses can be written into a single memory cell of the flash memory. The flash memory is a kind of a 1 cell 2 bit flash memory.
FIG. 1 shows a structure of conventional silicon nitride read only memory. The silicon nitride read only memory comprises a substrate 100, a silicon oxide/silicon nitride/silicon oxide (ONO) layer 102 on the substrate 100, and a gate conductive layer 110 on the ONO layer 102. The ONO layer 102 further comprises the silicon oxide layer as the tunneling oxide layer 104, the silicon nitride layer 106, and the silicon oxide layer 108 as the dielectric layer. The gate conductive layer 110 includes a polysilicon layer. A source region 112 and a drain region 114 are formed in the substrate 100 at two sides of the ONO layer 112. The silicon nitride read only memory further comprises a channel region 116 under the ONO layer 102 and between the source region 112 and the drain region 114. In addition, pocket implant regions 118 are formed in the substrate 100 between the ONO layer 102 and the drain region 114, and between the ONO layer 102 and the source region 112.
In the conventional silicon nitride read only memory, the source region 112, the drain region 114 are doped with the same conductive type, for example, P-type or N-type. Thus, a P-type silicon nitride read only memory has P-type source region 112, drain region 114. The pocket implant regions 118 are doped with N-type dopants to increase the PN junction concentration so as to increase the electric field.
In the typical process for fabricating the silicon nitride read only memory, due to the influence of the process environment, for example, using plasma to generate electrons from the substrate, some electrons 120 are stored in the silicon nitride layer 106 to cause a non-uniform program of the read only memory. The electrons 120 stored into the silicon nitride layer 106 are non-uniform and cause different initialization statuses for each device. In addition, the electrons stored in the silicon nitride layer 106 are difficult to eliminate and cause the difficulty when used for the first time. To solve the above problem, one can only avoid the injection of electrons into the silicon nitride layer 106. A good initialization method is still lacking.
The invention provides an initialization method of a P-type silicon nitride read only memory. After the P-type silicon nitride read only memory is fabricated, an ultra-violet light is uniformly radiated on the P-type silicon nitride read only memory. Consequently, a uniform distribution of electron traps is obtained in the silicon nitride layer of the P-type silicon nitride read only memory. The device is thus maintained at a status with a low threshold voltage (Low|Vt|) to achieve an initialization effect.
In the invention, using the radiation of an ultra-violet light, the device can obtain a unique threshold voltage Vt no matter what the process condition is.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.